An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of the duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (a steering handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. Further, in a vehicle with an idling stop function, the ignition key signal is inputted into the control unit 100 via a voltage stabilization circuit 30 and then through the ignition key 11. The control unit 100 calculates a steering assist command value of an assist (steering assist) command based on a steering torque Tr detected by the torque sensor 10 and a vehicle velocity Vel detected by a vehicle velocity sensor 12, and controls a current supplied to the motor 20 based on a current control value E obtained by performing compensation and so on with respect to the steering assist command value. Moreover, it is also possible to receive the vehicle velocity Vel from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tr detected by the torque sensor 10 and the vehicle velocity Vel from the vehicle velocity sensor 12 are inputted into a steering assist command value calculating section 101, and a steering assist command value Iref is calculated by means of an assist map. The calculated steering assist command value Iref is inputted into a maximum output limiting section 102 and an output is limited based on an overheat protection condition or the like in the maximum output limiting section 102. A current command value I that maximum output is limited, is inputted into a subtracting section 103.
Moreover, with respect to the calculation of the steering assist command value Iref performed in the steering assist command value calculating section 101, it is also possible to calculate the steering assist command value Iref by using not only the steering torque Tr and the vehicle velocity Vel but also a steering angle.
The subtraction section 103 calculates a deviation ΔI (=I−i) between the current command value I and a motor current i of the motor 20 that is fed back, the deviation ΔI is controlled by a current control section 104 such as a PI control (proportional and integral control) or the like, the controlled current control value E is inputted into a PWM (Pulse Width Modulation) control section 105 and the duty ratio is calculated, and in accordance with a PWM-signal PS that the duty ratio is calculated, the motor 20 is driven through a motor drive circuit 106. The motor current i of the motor 20 is detected by a motor current detecting circuit 107, and the detected motor current i is inputted into the subtracting section 103 to feed back.
A configuration example of the motor drive circuit 106 will be described with reference to FIG. 3. In the case of a three-phase motor, the motor drive circuit 106 comprises an FET gate drive circuit 106A that drives each gate of field-effect transistors (FET1 to FET6) based on the PWM-signal PS from the PWM control section 105, an inverter 106B comprising a three-phase bridge circuit of FET1 to FET6 and a booster circuit 106C that boosts high side FETs (FET1, FET2 and FET3). Further, with respect to FET1 to FET6, diodes D1 to D6 for surge absorbing are built between their respective sources and their respective drains in anti-parallel. Moreover, with respect to the high side FET1, FET2 and FET3, each pair of Zener diodes ZD1 to ZD3 for protecting gate is connected between their respective gates and their respective sources.
Electric power is supplied from the battery 13 as a power source to the inverter 106B through a power-source relay RL. The inverter 106B comprises an FET-array that the FET1 and the FET4 are connected in series, an FET-array that the FET2 and the FET5 are connected in series, and an FET-array that the FET3 and the FET6 are connected in series, and these three FET-arrays connected in series are connected in parallel. From a connecting point of the FET1 and the FET4 in the inverter 106B, a connecting point of the FET2 and the FET5 in the inverter 106B and a connecting point of the FET3 and the FET6 in the inverter 106B, each motor phase current is supplied to the motor 20 through supply routes “a”, “b” and “c”.
In such an electric power steering apparatus, the battery 13 supplies the electric power to loading apparatuses such as the control unit 100, the torque sensor 10, the motor 20 and so on. In order to assist steering operations of a driver to be stable normally, it is necessary to maintain the power-source voltage (the battery voltage) of the battery 13 in a given stable range (for example, 10V-15V). However, in a situation such as cranking, there is a possibility that the power-source voltage reduction occurs.
In a state that the power-source voltage dropped, the gate driving voltage of the FET used in the motor drive circuit 106 drops. In this case, when the voltage (VGS) from gate to source of the FET dropped, the drain-source on-state resistance (RDS (ON)) becomes large abruptly. For comparison, there is a relation such as the following Expression 1 between a maximum driving current
Imax and an allowable power value Pa of the FET.Pa=RDS(ON)·Imax2  [Expression 1]                where, “Pa” is the allowable power value of the FET, “RDS(ON)” is the drain-source on-state resistance of the FET, and “Imax” is a motor maximum current that can pass in the FET.        
From the relation of the above Expression 1, in the case of drive control of the motor 20, when the drain-source on-state resistance (RDS(ON)) of the FET becomes large, power loss also becomes large. Therefore, when the power-source voltage dropped, due to heat occurred by power loss of the FETs, the temperature increases. In addition, when the power-source voltage reduction continues, there is a possibility that a failure that the FET is damaged by burnout occurs.
Further, when the power-source voltage dropped dramatically and dropped to less than or equal to a minimum operating voltage of the torque sensor 10, the output of the torque sensor 10 descends, a neutral position of the steering wheel 1 becomes being off track, the current characteristic of the motor 20 also becomes being off track from the neutral position of the steering wheel 1. Therefore, there was such a problem, that is, a bilateral difference of the steering force of the steering wheel occurs, when the bilateral difference becomes abysmal, a trouble such as “the steering wheel is taken” occurs, the steering feeling becomes bad. That is to say, when the power-source voltage becomes less than or equal to a certain voltage value, the torque sensor 10 cannot work normally.
Accordingly, when the power-source voltage dropped, in order to keep a good steering feeling, it is necessary to limit or shut down the assist control. In order to solve such a problem, in Patent Document 1 (Japanese Published Unexamined Patent Application No.2005-193751 A), an electric power steering apparatus that limits the assist amount by means of a variable limitation value in accordance with the power-source voltage in the case of the power-source voltage drop, is proposed. Further, in Patent Document 2 (Japanese Published Unexamined Patent Application No. 2007-290429 A), an electric power steering apparatus that comprises semiconductor switching elements with a low on-state resistance at a time of low voltage, when the power-source voltage is more than or equal to a lower limit of the operation voltage, performs the control of the motor, and when the power-source voltage is less than the operation voltage, shuts down the control of the motor, is proposed.